The Impact of Mycorrhizosphere Bacterial
Chapter 5
The Impact of Mycorrhizosphere Bacterial
Communities on Soil Biofunctioning in Tropical
and Mediterranean Forest Ecosystems
Robin Duponnois, Ezekiel Baudoin, Jean Thioulouse, Mohamed Hafidi,
Antoine Galiana, Michel Lebrun, and Yves Prin
5.1 Introduction
Mycorrhizal fungi constitute a key functional group of soil biota that greatly
contribute to productivity and sustainability of terrestrial ecosystems. These are
ubiquitous components of most of the ecosystems throughout the world and are
considered key ecological factors in governing the cycles of major plant nutrients
and in sustaining the vegetation cover (van der Hejden eta!. 1998; Requena et al.
2001; Schreiner et al. 2003). Two major forms of mycorrhizae are usually
recognized: the arbuscular mycorrhiza (AM) and the ectomycorrhizas (ECMs).
AM symbiosis is the most widespread mycorrhizal association type with plants that
have true roots, i.e. pteridophytes, gymnosperms and angiosperms (Read eta!. 2000).
R. DU.ponnois (t8J) • E. Baudoin_ • M. Lebrun
IRD, UMR 113 CIRAD!INRA/IRD/SUP-AGRO(UM2, Laboratoire des Symbioses Tropicales
et M6diterran6ennes (LSTM), TA A-82/J, Campus International de Baillarguet, Montpellier
Cedex 5, France
Laboratoire Ecologie & Environnement (Unite associ6e au CNRST, URAC 32), Facult6 des
Sciences Semlalia, Universit6 Cadi Ayyad, Marrakech, Marocco
e-mail: Robin.Duponnois@ird.fr
J. Thioulouse
Universit6 de Lyon, 69000 Lyon, France
CNRS, UMR5558, Laboratoire de Biom6trie,et Biologie Evolutive, Universit6 Lyon 1, 69622
Villeurbanne, France
M. Hafidi
Laboratoire Ecologie & Environnement (Unit6 associ6e au CNRST, URAC 32), Facult6 des
Sciences Semlalia, Universit6 Cadi Ayyad, Marrakech, Marocco
A. Galiana • Y. Prin
CIRAD, UMR 113 CIRAD/INRA/IRD/SUP-AGRO/UM2, Laboratoire des Symbioses Tropicales
et Mediterraneennes (LSTM), TA A-82/J, Campus International de Baillarguet, Montpellier
Cedex 5, France
80
R. Duponnois et al.
5
They affect about 80-90% land plants in natural, agricultural and forest ecosystems
(Brundrett 2002). ECMs are found with trees and shrubs, gymnosperms (Pinaceae)
and angiosperms, and usually result from the association of homobasidiomycetes
with about 20 families of mainly woody plants (Smith and Read 2008), These
woody species are associated with a larger (compared to the AM symbiosis)
diversity of fungi, comprising 4,000-6,000 species, mainly Basidiomycetes and
Ascomycetes (Allen et aL 1995; Valentine et aL 2004). The benefits of mycorrhizal
symbiosis to the host plant have Usually been considered as a result of closed
relationships between the host plant and the fungal symbiont However, the hyphae
Increase of soil stability and
water retention
Increase of plant tolerance
against abiotic stresses
of these symbiotic fungi provide an increased area for interactions with other soil
microorganisms by enhancing the development of the host plant root systems.
Plant roots influence the soil microbial community in the narrow zone of soil
Increase of plant tolerance
against biotic stresses
called the rhizosphere (Hiltner 1904). In the rhitosphere, root exudates and organic
Increase plant quality for
human health
breakdown products provide a specific ecological niche for microbes with chemical
and physical characteristics (concentration and forms of nutrients, soil structure,
moisture and pH) that differ from those recorded in the bulk soil (Timonen and
Marschner 2006), Hence the density and activity of microorganisms are generally
higher in the rhizosphere than in the bulk soil (Lynch 1990). Since plant roots in
natural conditions are mycorrhizal and it is well known that the fungal symbiosis
81
The Impact of Mycorrhizosphere Bacterial Communities
Fig. 5.1 The Mycorrhizosphere trophic complex and its role as an ecosystem service provider
modifies root functions, microbial communities associated with mycorrhizas differ
from those of the non-mycorrhizal plants and of the surrounding soil (Garbaye
1991; Garbaye and Bowen 1987, 1989), Hence the rhizosphere concept has been
enlarged to include the fungal component of the symbiosis to give the term
"mycorrhizosphere" (Rarnbelli 1973; Linderman 1988). The mycorrhizosphere is
the zone influenced by both the root and themycorrhizal fungus. It includes the soil
surrounding individual fungal hyphae that has been named "hyphosphere"
(Johansson et aL 2004). Mycorrhizal fungi act as a bridge connecting the rhizosphere to the bulk soil and, through an active development of extraradical mycelium
into the soil, the mycorrhizosphere extends root-fungal interactions with soil
microbial communities (Whipps 2004; Leake et aL 2004 ). Interactions within the
mycorrhizosphere microbial community are of special interest because some
microorganisms · associated with mycorrhiza may complement mycorrhizal
activities (Toro et aL 1996), More recently, Frey-Klett et aL (2005) have proposed
that the ectomycorrhizal symbiosis could be considered as a microbial complex
where the fungal symbiosis has a direct effect on plant growth (nutritional and
hormonal mechanisms) but also an indirect positive effect via a selective pressure
on bacterial communities resulting, for instance, to a higher abundance of
phosphate-solubilizing fluorescent pseudomonads in the hyphosphere. It has been
previously demonstrated that P-solubilizing bacteria can interact synergistically
with mycorrhizal fungi and improve the phosphorus nutrition of the host plant
(Muthukumar et aL 2001),
consequences on soil biofunctioning and ecosystem productivity remains poorly
understood (Fig. 5.1). On one hand, some soil bacteria can act as mycorrhization
helper bacteria (MHB) by improving the establishment of the mycorrhizal symbiosis and on the other hand, mycorrhizal fungi can have an impact on the structure and
functional diversity of bacterial communities (Assigbetse et aL 2005; Artursson et aL
2005). The purpose of this chapter is to· outline the mycorrhizosphere interactions
betwee;n ectomycorrhizal ｾｵｮｧｩ＠
associated with forest tree species and soil microflora
of potentially synergistic properties that lead to stimulation of plant growth. By
focussing on the ectomycorrhizal symbiosis associated with Tropical and Mediterranean tree species, we will review the global effects of ectomycorrhizal symbiosis on
the functional diversity of soil microftora and in particular, the interactions between
ectomycorrhizal fungi and some plant-growth-promoting rhizobacteria (i.e. rhizobia,
fluorescent pseudomonads). It is well known that ectomycorrhizal fungi improve the
phosphorus uptake of their associated host plant and enhance the plant development
(Read and Perez-Moreno 2003). This ectomycorrhizal effect on plant growth has
been mainly ascribed to the fact that the extramatrical mycelium increased the
abilities of the host plant to explore a larger volume of soil than roots alone and to
uptake nutrients froin a greater surface area through different biological processes
(Srnith and Read 2008). Phosphorus (P) is one of the most essential macronutrients
required for the growth and development of plant (lllmer and Schirmer 1992) and
occurs in various organic and- inorganic forms not directly assimilable by plants
To date, there is little information on the mechanisms controlling interactions
between mycorrhizal fungi, soil bacteria and plant roots in the mycorrhizosphere.
and soil microorganisms. In degraded areas, the first objective of the controlled
Although these interactions can influence the fungal symbiont itself and the plant
mycorrhization processes is to improve reforestation in areas presenting a loss or
v
w
82
R. Duponnois et al.
reduction of major physico-chemical and biological soil properties (Requena et al.
2001) and more particularly severe phosphorus deficienCies. This review aimed to
state the interactions between ectomycorrhizal fungi and soil microflora leading to
a sustainable microbial complex with high efficiency against phosphorus mobiliza-
tion and transferring phosphorus from the soil organic matter (SOM) or from soil
minerals to the host plant.
5
The Impact of Mycorrhizosphere Bacterial Communities
from soil solution through the production of extracellular acid phosphatase
(Quiquampoix and Mousain 2005). This beneficial effect (enhancement P nutrition
of the host plant) is generally attributed to the development of external hyphae into
the soil resulting in higher of soil volume exploited compared with nonmycorrhizal
roots (Louche eta!. 2010).
5.2.2
5.2
Soil Microbial Processes Involved in Phosphorus
Mobilization from Soil Organic Matter and Soil Minerals
Ectomycorrhizal fungi enhance the capacity
oi the host plants to mobilize P from
soil inorganic and organic forms through different biological processes that are
summarized below.
5.2.1
Mobilization of P from Soil Organic Matter
Soil organic matter contains a wide range of complex molecules such as inositol
phosphate,' nucleotides and phospholipids (Criquet eta!. 2004). Soil microbes can
degrade P-compounds through their capacity to produce a wide range of extracellular and surface-bound enzymes leading to the release into the soil of smaller
organic compounds that provide potential Sources of P for plants, ectomycorrhizal
fungi and other soil microorganisms (Nahas eta!. 1982; Haas eta!. 1992). Phosphatase activities (acid and alkaline phosphatase) release orthophosphate ions (Pi)
that are the unique form of P easily assimilable by soil microorganisms and plants
(Rao eta!. 1996). Enzymes can be free in the soil solution or bound to soil colloids,
to humic substances, to living and dead microbial cells or to plant roots. Acid
phosphatase activity in the rhizosphere may also be due to plant roots (Goldstein
eta!. 1988; Coello 2002), bacteria (Palacios eta!. 2005; Boyce and Walsh 2007)
and fungi (Yoshida eta!. 1989; Weber and Pitt 1997; Bernard eta!. 2002). The
secretion of acid phosphatases is induced by Pi-deficient conditions for all the
organisms studied (Goldstein et a!. 1988; Bernard et a!. 2002). Most of the studies
83
Mobilization of P from Soil Minerals
Ectomycorrhizal fungi have the potential ability to mobilize and translocate essential plant nutrients from minerals (Landeweert eta!. 2001). Weathering processes
of minerals (transformation of rock-forming primary minerals into dissolved
compounds and secondary mineral residues into the biological environment) result
from the activity of plant roots and their associated microbiota (rhizosphere bacteria
and fungi). Plant root exudates and root-associated microorganisms affect the
stability of minerals through the production of organic acids, phenolic compounds,
protons, siderophores and polysaccharides (Barker eta!. 1997; Drever 1994; Drever
and Vance 1994). Soluble organic acids affecting mineral weathering range from
low to high molecular weight such as humic substances but low molecular weight
(LMW) organic acids are considered to be the main agents of mineral dissolution
because of their dual acidifying and complexing properties (Ochs 1996; Barker
et a!. 1998). Numerous studies have shown that ectomycorrhizal fungal strains
could dissolve minerals by excreting organic acids. Among these organic acids
excreted by ectomycorrhizal fungi, oxalate, citrate and malate are the strongest
chelators of trivalent metals. Oxalic acid is known to have the highest acid strength
(Gadd 1999). This organic acid is involved in the dissolution process of common
soil minerals such as apatite, biotite, phlogopite and microline (Courty eta!. 201 0).
Many ectomycorrhizal fungi excrete oxalic acid in pure culture (Paris eta!. 1996),
but this organic acid excretion is also observed with ectomycorrhizas (van Scholl
et a!. 2006) and hypha! mats (Wallander et a!. 2003). However, most of these
studies have been performed in pure cultures or in pot experiments and the real
contribution of ectomycorrhizal fungi in mineral weathering remains difficult to
determine in natural conditions (Landeweert eta!. 2001; Courty eta!. 2010).
performed in laboratory conditions with known substrates showed that these enzy-
matic activities are generally not substrate specific, except for phytases (Wyss eta!.
1999), and are able to release Pi from different phosphorylated substrates. Phosphatase activity has often been used as a general indicator in measurements of
5.3
Impact of the Controlled Ectomycorrhization on Soil
Microbial Functions and Phosphorus Mobilization
biological activity (Joner and Johansen 2000). Ectomycorrhizal fungi differ in their
physiological capacities to acquire and transfer nutrients to a range of plant hosts
In forest formations, extend of extraradical mycelium of ectomycorrhizas intercon-
(Abuzinadah and Read 1989; Dighton et a!. 1993; Bending and Read 1995). It has
been suggested that ectomycorrhizal fungi contribute to organic P mobilization
nect roots belonging to the same or different ECM tree species. This common
mycorrhizal network (CMN) allows a transfer of C and nutrients between host
84
R. Duponnois et al.
5
0.6
plants (Simard and Durall 2004). Hence, the ability of ectomycorrhizal fungi to
mobilize phosphorus and to transfer Pi to the host plants through this common
mycorrhizal network could be supplemented by their positive selective pressure on
soil microbial functions (i.e. phosphatase activity). This CMN effect is of particular
importance in Mediterranean and Tropical areas where it has been clearly
demonstrated that land degradation is associated with reductions in the below
ground microbial diversity and/or activity (Kennedy and Smith 1995; Garcia
et a!. 1997).
The functioning of soil microbial community is central to understand ecosystemlevel processes such as decomposition and nutrient cycling. Various standardized
methodologies have been developed to determine the microbial functional
characteristics (i.e. enzymatic activities, BiologTM method, etc.). Degens and Harris
( 1997) proposed a method that avoids the pr
The Impact of Mycorrhizosphere Bacterial
Communities on Soil Biofunctioning in Tropical
and Mediterranean Forest Ecosystems
Robin Duponnois, Ezekiel Baudoin, Jean Thioulouse, Mohamed Hafidi,
Antoine Galiana, Michel Lebrun, and Yves Prin
5.1 Introduction
Mycorrhizal fungi constitute a key functional group of soil biota that greatly
contribute to productivity and sustainability of terrestrial ecosystems. These are
ubiquitous components of most of the ecosystems throughout the world and are
considered key ecological factors in governing the cycles of major plant nutrients
and in sustaining the vegetation cover (van der Hejden eta!. 1998; Requena et al.
2001; Schreiner et al. 2003). Two major forms of mycorrhizae are usually
recognized: the arbuscular mycorrhiza (AM) and the ectomycorrhizas (ECMs).
AM symbiosis is the most widespread mycorrhizal association type with plants that
have true roots, i.e. pteridophytes, gymnosperms and angiosperms (Read eta!. 2000).
R. DU.ponnois (t8J) • E. Baudoin_ • M. Lebrun
IRD, UMR 113 CIRAD!INRA/IRD/SUP-AGRO(UM2, Laboratoire des Symbioses Tropicales
et M6diterran6ennes (LSTM), TA A-82/J, Campus International de Baillarguet, Montpellier
Cedex 5, France
Laboratoire Ecologie & Environnement (Unite associ6e au CNRST, URAC 32), Facult6 des
Sciences Semlalia, Universit6 Cadi Ayyad, Marrakech, Marocco
e-mail: Robin.Duponnois@ird.fr
J. Thioulouse
Universit6 de Lyon, 69000 Lyon, France
CNRS, UMR5558, Laboratoire de Biom6trie,et Biologie Evolutive, Universit6 Lyon 1, 69622
Villeurbanne, France
M. Hafidi
Laboratoire Ecologie & Environnement (Unit6 associ6e au CNRST, URAC 32), Facult6 des
Sciences Semlalia, Universit6 Cadi Ayyad, Marrakech, Marocco
A. Galiana • Y. Prin
CIRAD, UMR 113 CIRAD/INRA/IRD/SUP-AGRO/UM2, Laboratoire des Symbioses Tropicales
et Mediterraneennes (LSTM), TA A-82/J, Campus International de Baillarguet, Montpellier
Cedex 5, France
80
R. Duponnois et al.
5
They affect about 80-90% land plants in natural, agricultural and forest ecosystems
(Brundrett 2002). ECMs are found with trees and shrubs, gymnosperms (Pinaceae)
and angiosperms, and usually result from the association of homobasidiomycetes
with about 20 families of mainly woody plants (Smith and Read 2008), These
woody species are associated with a larger (compared to the AM symbiosis)
diversity of fungi, comprising 4,000-6,000 species, mainly Basidiomycetes and
Ascomycetes (Allen et aL 1995; Valentine et aL 2004). The benefits of mycorrhizal
symbiosis to the host plant have Usually been considered as a result of closed
relationships between the host plant and the fungal symbiont However, the hyphae
Increase of soil stability and
water retention
Increase of plant tolerance
against abiotic stresses
of these symbiotic fungi provide an increased area for interactions with other soil
microorganisms by enhancing the development of the host plant root systems.
Plant roots influence the soil microbial community in the narrow zone of soil
Increase of plant tolerance
against biotic stresses
called the rhizosphere (Hiltner 1904). In the rhitosphere, root exudates and organic
Increase plant quality for
human health
breakdown products provide a specific ecological niche for microbes with chemical
and physical characteristics (concentration and forms of nutrients, soil structure,
moisture and pH) that differ from those recorded in the bulk soil (Timonen and
Marschner 2006), Hence the density and activity of microorganisms are generally
higher in the rhizosphere than in the bulk soil (Lynch 1990). Since plant roots in
natural conditions are mycorrhizal and it is well known that the fungal symbiosis
81
The Impact of Mycorrhizosphere Bacterial Communities
Fig. 5.1 The Mycorrhizosphere trophic complex and its role as an ecosystem service provider
modifies root functions, microbial communities associated with mycorrhizas differ
from those of the non-mycorrhizal plants and of the surrounding soil (Garbaye
1991; Garbaye and Bowen 1987, 1989), Hence the rhizosphere concept has been
enlarged to include the fungal component of the symbiosis to give the term
"mycorrhizosphere" (Rarnbelli 1973; Linderman 1988). The mycorrhizosphere is
the zone influenced by both the root and themycorrhizal fungus. It includes the soil
surrounding individual fungal hyphae that has been named "hyphosphere"
(Johansson et aL 2004). Mycorrhizal fungi act as a bridge connecting the rhizosphere to the bulk soil and, through an active development of extraradical mycelium
into the soil, the mycorrhizosphere extends root-fungal interactions with soil
microbial communities (Whipps 2004; Leake et aL 2004 ). Interactions within the
mycorrhizosphere microbial community are of special interest because some
microorganisms · associated with mycorrhiza may complement mycorrhizal
activities (Toro et aL 1996), More recently, Frey-Klett et aL (2005) have proposed
that the ectomycorrhizal symbiosis could be considered as a microbial complex
where the fungal symbiosis has a direct effect on plant growth (nutritional and
hormonal mechanisms) but also an indirect positive effect via a selective pressure
on bacterial communities resulting, for instance, to a higher abundance of
phosphate-solubilizing fluorescent pseudomonads in the hyphosphere. It has been
previously demonstrated that P-solubilizing bacteria can interact synergistically
with mycorrhizal fungi and improve the phosphorus nutrition of the host plant
(Muthukumar et aL 2001),
consequences on soil biofunctioning and ecosystem productivity remains poorly
understood (Fig. 5.1). On one hand, some soil bacteria can act as mycorrhization
helper bacteria (MHB) by improving the establishment of the mycorrhizal symbiosis and on the other hand, mycorrhizal fungi can have an impact on the structure and
functional diversity of bacterial communities (Assigbetse et aL 2005; Artursson et aL
2005). The purpose of this chapter is to· outline the mycorrhizosphere interactions
betwee;n ectomycorrhizal ｾｵｮｧｩ＠
associated with forest tree species and soil microflora
of potentially synergistic properties that lead to stimulation of plant growth. By
focussing on the ectomycorrhizal symbiosis associated with Tropical and Mediterranean tree species, we will review the global effects of ectomycorrhizal symbiosis on
the functional diversity of soil microftora and in particular, the interactions between
ectomycorrhizal fungi and some plant-growth-promoting rhizobacteria (i.e. rhizobia,
fluorescent pseudomonads). It is well known that ectomycorrhizal fungi improve the
phosphorus uptake of their associated host plant and enhance the plant development
(Read and Perez-Moreno 2003). This ectomycorrhizal effect on plant growth has
been mainly ascribed to the fact that the extramatrical mycelium increased the
abilities of the host plant to explore a larger volume of soil than roots alone and to
uptake nutrients froin a greater surface area through different biological processes
(Srnith and Read 2008). Phosphorus (P) is one of the most essential macronutrients
required for the growth and development of plant (lllmer and Schirmer 1992) and
occurs in various organic and- inorganic forms not directly assimilable by plants
To date, there is little information on the mechanisms controlling interactions
between mycorrhizal fungi, soil bacteria and plant roots in the mycorrhizosphere.
and soil microorganisms. In degraded areas, the first objective of the controlled
Although these interactions can influence the fungal symbiont itself and the plant
mycorrhization processes is to improve reforestation in areas presenting a loss or
v
w
82
R. Duponnois et al.
reduction of major physico-chemical and biological soil properties (Requena et al.
2001) and more particularly severe phosphorus deficienCies. This review aimed to
state the interactions between ectomycorrhizal fungi and soil microflora leading to
a sustainable microbial complex with high efficiency against phosphorus mobiliza-
tion and transferring phosphorus from the soil organic matter (SOM) or from soil
minerals to the host plant.
5
The Impact of Mycorrhizosphere Bacterial Communities
from soil solution through the production of extracellular acid phosphatase
(Quiquampoix and Mousain 2005). This beneficial effect (enhancement P nutrition
of the host plant) is generally attributed to the development of external hyphae into
the soil resulting in higher of soil volume exploited compared with nonmycorrhizal
roots (Louche eta!. 2010).
5.2.2
5.2
Soil Microbial Processes Involved in Phosphorus
Mobilization from Soil Organic Matter and Soil Minerals
Ectomycorrhizal fungi enhance the capacity
oi the host plants to mobilize P from
soil inorganic and organic forms through different biological processes that are
summarized below.
5.2.1
Mobilization of P from Soil Organic Matter
Soil organic matter contains a wide range of complex molecules such as inositol
phosphate,' nucleotides and phospholipids (Criquet eta!. 2004). Soil microbes can
degrade P-compounds through their capacity to produce a wide range of extracellular and surface-bound enzymes leading to the release into the soil of smaller
organic compounds that provide potential Sources of P for plants, ectomycorrhizal
fungi and other soil microorganisms (Nahas eta!. 1982; Haas eta!. 1992). Phosphatase activities (acid and alkaline phosphatase) release orthophosphate ions (Pi)
that are the unique form of P easily assimilable by soil microorganisms and plants
(Rao eta!. 1996). Enzymes can be free in the soil solution or bound to soil colloids,
to humic substances, to living and dead microbial cells or to plant roots. Acid
phosphatase activity in the rhizosphere may also be due to plant roots (Goldstein
eta!. 1988; Coello 2002), bacteria (Palacios eta!. 2005; Boyce and Walsh 2007)
and fungi (Yoshida eta!. 1989; Weber and Pitt 1997; Bernard eta!. 2002). The
secretion of acid phosphatases is induced by Pi-deficient conditions for all the
organisms studied (Goldstein et a!. 1988; Bernard et a!. 2002). Most of the studies
83
Mobilization of P from Soil Minerals
Ectomycorrhizal fungi have the potential ability to mobilize and translocate essential plant nutrients from minerals (Landeweert eta!. 2001). Weathering processes
of minerals (transformation of rock-forming primary minerals into dissolved
compounds and secondary mineral residues into the biological environment) result
from the activity of plant roots and their associated microbiota (rhizosphere bacteria
and fungi). Plant root exudates and root-associated microorganisms affect the
stability of minerals through the production of organic acids, phenolic compounds,
protons, siderophores and polysaccharides (Barker eta!. 1997; Drever 1994; Drever
and Vance 1994). Soluble organic acids affecting mineral weathering range from
low to high molecular weight such as humic substances but low molecular weight
(LMW) organic acids are considered to be the main agents of mineral dissolution
because of their dual acidifying and complexing properties (Ochs 1996; Barker
et a!. 1998). Numerous studies have shown that ectomycorrhizal fungal strains
could dissolve minerals by excreting organic acids. Among these organic acids
excreted by ectomycorrhizal fungi, oxalate, citrate and malate are the strongest
chelators of trivalent metals. Oxalic acid is known to have the highest acid strength
(Gadd 1999). This organic acid is involved in the dissolution process of common
soil minerals such as apatite, biotite, phlogopite and microline (Courty eta!. 201 0).
Many ectomycorrhizal fungi excrete oxalic acid in pure culture (Paris eta!. 1996),
but this organic acid excretion is also observed with ectomycorrhizas (van Scholl
et a!. 2006) and hypha! mats (Wallander et a!. 2003). However, most of these
studies have been performed in pure cultures or in pot experiments and the real
contribution of ectomycorrhizal fungi in mineral weathering remains difficult to
determine in natural conditions (Landeweert eta!. 2001; Courty eta!. 2010).
performed in laboratory conditions with known substrates showed that these enzy-
matic activities are generally not substrate specific, except for phytases (Wyss eta!.
1999), and are able to release Pi from different phosphorylated substrates. Phosphatase activity has often been used as a general indicator in measurements of
5.3
Impact of the Controlled Ectomycorrhization on Soil
Microbial Functions and Phosphorus Mobilization
biological activity (Joner and Johansen 2000). Ectomycorrhizal fungi differ in their
physiological capacities to acquire and transfer nutrients to a range of plant hosts
In forest formations, extend of extraradical mycelium of ectomycorrhizas intercon-
(Abuzinadah and Read 1989; Dighton et a!. 1993; Bending and Read 1995). It has
been suggested that ectomycorrhizal fungi contribute to organic P mobilization
nect roots belonging to the same or different ECM tree species. This common
mycorrhizal network (CMN) allows a transfer of C and nutrients between host
84
R. Duponnois et al.
5
0.6
plants (Simard and Durall 2004). Hence, the ability of ectomycorrhizal fungi to
mobilize phosphorus and to transfer Pi to the host plants through this common
mycorrhizal network could be supplemented by their positive selective pressure on
soil microbial functions (i.e. phosphatase activity). This CMN effect is of particular
importance in Mediterranean and Tropical areas where it has been clearly
demonstrated that land degradation is associated with reductions in the below
ground microbial diversity and/or activity (Kennedy and Smith 1995; Garcia
et a!. 1997).
The functioning of soil microbial community is central to understand ecosystemlevel processes such as decomposition and nutrient cycling. Various standardized
methodologies have been developed to determine the microbial functional
characteristics (i.e. enzymatic activities, BiologTM method, etc.). Degens and Harris
( 1997) proposed a method that avoids the pr